There is great enthusiasm by some for surgical reconstruction of the Anterior Cruciate Ligament (ACL) of the knee. Many who have lost their ACL go on to develop osteoarthrosis. The paradox is that others, following ACL rupture, continue to function without problems, even at the professional athletic level. In addition, although meniscal damage is more common following ACL loss, its role in the pathophysiology of osteoarthrosis after ACL rupture is unclear. We hypothesize that the degree of dynamic instability of the knee after ACL loss varies from individual to individual, and is correlated to the degree of subsequent meniscal damage and the development of osteoarthrosis. Using a canine model of ACL transection, the time history of events which follow ACL loss will be documented to define the relationships between dynamic knee instability, meniscal breakdown, and osteoarthrosis. Knee dynamic instability will be assessed during gait by three dimensional photogrammetric analysis based on a unique high-speed biplanar digital radiography system. Articular and meniscal changes will be determined with MRI. Kinematic and MRI testing will be performed on each of 25 dogs before and serially for two years after ACL transection. The goal of this project is to clarify the importance of altered mechanics on the development of osteoarthrosis of the knee following ACL loss. These findings should have significant clinical implications upon the choice of appropriate care of individuals following ACL injury, creating a rational predictive basis for when to surgically intervene.